DS-CDMA or CDMA communication systems are being implemented as cellular telephone systems. A CDMA system includes a system controller and at least one base station. Each base station provides communication service to a fixed geographic area or cell. Mobile stations in a cell communicate with the base station for that cell. Communication with a mobile station is handed off among base stations as the mobile station moves among cells. One example of such a system is a system according to EIA/TIA interim standard 95 Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System ("IS-95").
In CDMA systems including IS-95 systems, the communication link between mobile station and base station includes a forward link from base station to mobile station and a reverse link from mobile station to base station. A mobile station, as a consumer product, is tightly limited in its cost of materials and manufacturing cost. Therefore, the mobile station commonly omits redundant hardware components and other features. For example, such mobile stations have not used space diversity techniques to increase receiver sensitivity because to do so requires two antennas and two radio frequency (RF) signal processing paths in the mobile station as well as a diversity control circuit. Since a base station is not as limited in its cost, base stations typically employ such diversity techniques and others to improve receiver sensitivity. This results in a base station receiver which is more sensitive than the mobile station receiver and an unbalanced communication link.
One known DS-CDMA system provides a path diversity function. The base station includes multiple antennas each with an associated delay device. The time delays provide diversity operation in the base station. Nevertheless, this prior system is severely limited to use in applications in which the communications channel exhibits low delay spread, such as in-building, microcellular, wireless private branch exchange (PBX) systems. In channels with significant delay spread, the performance of the prior system is degraded as added, delayed rays can not be used to advantage but merely increase the level of intracell interference at the receiver.
Accordingly, there is a need in the art for an improved apparatus and method for mobile station receiver diversity in a DS-CDMA system.